Liquid level meter

ABSTRACT

A method and apparatus for determining the mean liquid level in a container subject to motion wherein a signal related to the pressure exerted by the liquid near the bottom of the container is divided by a signal representing the acceleration forces on the container to provide a signal related to the mean liquid level in the container.

limited Mates Patent Hop [54] LIQUID LEVEL METER [72] Inventor: AdrianusG. 1110p, Amsterdam, Netherlands [73] Assignee: Shell Oil Company, NewYork, N.Y.

[22] Filed: Dec. 8, 1969 [21] Appl. No.: 883,211

[ 5] Feb. 8, 1972 3,092,916 6/1963 Kendziorek et al ..73/301 X 3,358,50912/1967 Edwards et a1. ....73/301 3,439,539 4/1969 Pallis ..73/301Primary ExaminerLouis R. Prince Assistant Examiner-Frederick ShoonAttorney-J. H. McCarthy and Theodore E. Bieber [5 7] ABSTRACT A methodand apparatus for determining the mean liquid level in a containersubject to motion wherein a signal related to the pressure exerted bythe liquid near the bottom of the container is divided by a signalrepresenting the acceleration forces on the container to provide asignal related to the mean liquid level in the container,

5 Claims, 4 Drawing Figures 5 l MASS I PRESSURE GAUGE Y i L DIFFERENCECIRCUIT 4 PRESSURE GAUGE a if n mvwmc cmcun LIQUID LEVEL METER Thepresent invention relates to a process and an apparatus for continuouslydetermining the mean level of a liquid present in a container which canmove.

The invention is of importance in determining the liquid level in steamboilers, fuel tanks, drinking water tanks used on ships, etc. On accountof the movements of the ship, the liquid surface is moving continuously,so that the measurement by known means likewise fluctuates continuously.For this reason, the measurement becomes inaccurate and of little value.

Another adverse effect occurs, for instance, in the process of automaticcontrol of the liquid level in a steam boiler on board a ship. In thisprocess the water supply to the boiler is controlled on the basis ofdata provided by a liquid level gauge. On account of the fluctuations inthe result of the measurement, the water supply also shows fluctuations.This renders it impossible to make optimum use of the steam boiler,unless, one exercises regular supervision and intervenes by manualoperation. However, recent technical developments show a trend towardsfurther automation and remote control of the machinery, as a result ofwhich operating and supervisory crew need no longer be regularly presentin the engine room.

BRIEF SUMMARY OF THE INVENTION The invention provides a way in which areliable measurement of the mean liquid level in boilers and similarapparatuses can be effected under the conditions mentioned.

The invention relates to a process for continuously determining the meanlevel of a liquid present in a container which can move, in whichprocess:

a. the mean pressure exerted by the liquid in the vertical directionnear the bottom of the container is determined and converted to acorresponding signalf,,,

b. the force exerted by a constant mass in the vertical direction, whichmass makes the same movements as the container is determined andconverted to a corresponding signal fc c. from signals f and f a signalf f /f, is derived, which signal f,, is a measure of the mean liquidlevel in the container.

Hereinafter, the process and apparatus will be described with referenceto the movements of a ship. However, the invention is by no meansrestricted to these since similar movements may occur in aircraft and infloating storage tanks. Further, the term pressure is used to define theforce per unit area.

The sea causes a ship to make tilting movements while the surface of aliquid in a container tries to remain horizontal, as a result of whichit will occupy varying slanting positions with respect to the container.A ship also makes movements in the vertical direction and as a result,the liquid in a container is subjected to accelerations anddecelerations, which are superimposed on the acceleration due togravity. In the process according to the invention both these influenceson the measurement are corrected for automatically. The process startsby determining the mean liquid pressure as mentioned, the result ofwhich determination depends on the weight of a column of liquid ofaverage height and on the accelerations or decelerations to which thiscolumn is subjected in the vertical direction. The liquid pressuresignal f therefore is a pressure signal which is not corrected for theinfluences of vertical movements. It is, however, corrected for theinfluences of tilt ing movements, as explained hereinafter.

The measurement of the vertical movement yields a signal f by means ofwhich the earlier obtained signal f, can be corrected for the influencesof vertical movements. The quotient f,, of f, divided by f when relatedto the mean level of the liquid.

In many cases the mean pressure can be determined by measuring thedifference between the pressure in a single point located at a low levelin the center of the container and the pressure in a point in thecontainer located above the surface of the liquid. This possibilitypresents itself in a container which has a horizontal cross section witha point of symmetry in the center. It is assumed that the liquid surfaceremains flat, or that, at least, the distribution of liquid levels alonglines through the point of symmetry is symmetrical in respect to thatpoint. By measuring the difference in pressure as indicated, theinfluence of the gas pressure is eliminated.

It is often to be preferred to employ a process in which the meanpressure mentioned under (a) is derived'from pressures measured inpoints located at a low level in the container, in the proximity of twoopposite, upright bounding walls of the containenand from the pressurein a point in the container located above the surface of the liquid.From the engineering point of view, it is not always possible to makemeasurements in the center of the container. The installation of gaugesor, at least, of connections in the proximity of upright bounding wallspresents no particular problems. From the pressures measured in twoopposite points as described hereinbefore the mean pressure can bederived by addition and division by two. In this way, again, the signalf,, is obtained.

The two opposite, upright bounding walls preferably are chosen as thoseat which the liquid in the container rises highest on account of themovements of the container. In that case the extreme liquid levels aretaken into account in the determination of the mean liquid pressure.Mostly these two upright bounding walls will be those with the greatestmutual distance.

It is also possible to measure the liquid pressure in more than twopoints in the container and to derive the signal f, from the dataobtained. One may, for instance, make measurements in the proximity ofeach upright bounding wall, which will usually mean that measurementsare made in four points. In that case, when the container makes veryirregular movements, a more accurate mean liquid pressure can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of oneembodiment of the invention;

FIG. 2 is a schematic drawing of a modified form of the in vention usingtwo gauges to measure the pressure;

FIG. 3 is a schematic drawing of a further modification of the inventionusing two differential pressure gauges to measure the pressure; and

FIG. 4 is a schematic drawing of an embodiment using two gauges tomeasure the pressure and circuits to compute the differential pressure.

An apparatus suitable for carrying out the process according to theinvention comprises:

d. at least one pressure gauge connected to a point or points located ata low level in the container,

e. a pressure gauge connected to a point in the container located abovethe surface of the liquid,

f. a constant mass connected to a pressure gauge for measuring thepressure exerted by the mass in the vertical direction, which results ina corresponding signal f g. a computing element for determining thesignal f,,, which corresponds to the mean pressure, from the signalsobtained by means of the gauges mentioned under (d) and (e),

h. a computing element for determining the signal f,,=f,,/f from thesignal obtained by means of the gauge mentioned under (f) and the signalf, mentioned under (g).

The gauge or gauges mentioned under (d) may be combined with the gaugementioned under (e) into a differential pressure gauge or differentialpressure gauges. In many cases preferably two differential pressuregauges will be employed, each with one input connected to a pointlocated at a low level in the container in the proximity of,respectively, an upright bounding wall and an upright bounding wallopposite the former wall, and with the other input connected to a pointin the container located above the surface of the liquid.

The constant mass mentioned hereinbefore under (f) may be a piece ofsolid matter or a liquid column. The force exerted in the verticaldirection can be measured by means of a balance or a pressure cell. Themagnitude of the mass is adapted to the measuring equipment to be used.The magnitude may,for instance, be 1 kg.

The apparatus may further comprise a computing element for determiningthe signal f =7",-l-f .f /n, wheref f .f, are the output signals of thedifferential pressure gauges, which are n in number, and a computingelement for determining the signalf, is always available. It can be madevisible, recorded, and used for control purposes.

The computations may also be made by digital methods after conversion ofthe gauge signals into the digital form. The signal f obtained in thisway can also be made visible, recorded, and used for control purposes,by known techniques.

In all figures, 1 represents the container, which is partly filled withliquid 2. The liquid level is drawn oblique wit respect to the containerin order to represent an arbitrary position of the latter. A constantmass 3 is connected to a pressure gauge 4, which is capable of measuringthe force exerted by mass 3 in the vertical direction. Mass 3 makes thesame movements as container 1.

In FIG. 1 in the center ofthe container a point is located at a lowlevel, in which point the pressure is measured with gauge 5. Gauge 6 isconnected to a point located at a high level in the container, above thesurface of the liquid.

The signals from gauges 5 and 6 pass to a computing element 7, wheresubtraction takes place to obtain the signal f The signal f and thesignal f from gauge 4 passes to a computing element 8, where divisiontakes place, resulting in the required signalf In FIG. 2 the pressure ismeasured by means of gauges 9 and 10 in two points located at a lowlevel in the container, near two opposite, upright bounding walls. Gauge1]. corresponds to gauge 6 in FIG. 1. The signals from gauges 9 and 10pass to computing element 12, where addition and division by two takeplace. In computing element 13 the signal from gauge 11 is subtractedfrom the result thus obtained, which results in the signalf From thissignal the signalf, is produced by computing element 14, in a mannersimilar to that outlined with respect to element 8 in FIG. 1.

In FIG. 3 the gauges l5 and 16 are differential pressure gaugessupplying output signals which have been compensated for the affect ofthe gas pressure. In computing element 17 the signals are added and thesum is divided by two. The signal f thus obtained passes to computingelement 18, which produces the signalf In FIG. 4 an alternative isrepresented to the embodiment comprising three pressure gauges, gauges19, 20 and 21 in this Figure corresponding to gauges 9, l0 and 11respectively, in FIG. 2. In computing element 22 the signal from gauge21 is subtracted from that from gauge 19, in computing element 23 thesignal from gauge 21 is subtracted from that from gauge 20. Thedifference signals thus obtained pass to computing element 24, whereaddition and division by two take place. Computing element 25 has thesame function as the elements 8, l4 and I8 mentioned hereinbefore.

A calculation on the basis of the diagram in FIG. 3 shows the following.

The acceleration due to gravity is g and the acceleration acting onliquid 2 and mass 3 is g'=g+a, where a is the acceleration-positive ornegative-due to the vertical component of the movement ofthe ship.

The differential pressure gauge supplies a signal corresponding tof=Apgh,

where A a constant p density ofthe liquid h height of the liquid abovethe lower measuring point of gauge 15.

Likewise, differential pressure gauge 16 supplies a signal correspondingtof =Apg'h where h height of the liquid above the lower measuring pointofgauge 16.

In computing element 17 f =(f +f )/2 is calculated. Pressure gauge 4supp res a signal corresponding to f B( m/s)gbhl where B a constant mmass of mass 3 s surface by which mg can be divided for calculating thepressure exerted by mass 3.

In computing element l8,f,, lf is calculated.

From the results obtained earlier it follows that Lo an/eg tea: C ma fIn Bm 2 2 where C is a constant.

It follows thatf is the signal required.

For the other diagrams provided corresponding calculations can be made.

I claim as my invention:

1. A process for continuously determining the mean le el of a liquidpresent in a container which can move, comprising:

a. measuring the mean pressure exerted by the liquid in the verticaldirection near the bottom of the container and converting it to acorresponding signal f wherein the mean pressure is determined bymeasuring the difference between the pressure in a single point locatedat a low level in the center of the container and the pressure in apoint in the container located above the surface of the liquid;

b. measuring the force exerted by a constant mass in the verticaldirection, which mass makes the same movements as the container,converting the measured corresponding signal f dividing the signal f, bysignal f, to obtain a signal related to the mean level of the liquid inthe container.

2. A process according to claim 1, in which process the mean pressure isderived from pressures measured in points located at a low level in thecontainer, in the proximity of two opposite, upright bounding walls ofthe container, and from the pressure in a point in the container locatedabove the surface of the liquid wherein the two opposite uprightbounding walls are those at which the liquid in the container riseshighest as a result of the movements of the container.

3. An apparatus for determining the mean level of a liquid in acontainer subject to motion comprising:

a. at least one pressure gauge connected to a point located at a lowlevel in the container;

b. at least one pressure gauge connected to a point in the containerlocated above the surface of the liquid,

c. a constant mass connected to a pressure gauge for measuring thepressure exerted by the mass in the vertical direction, which results ina corresponding signalf d. a computing element for determining thedifference between the measurements made by the first and secondpressure gauges to produce signal f e. a computing circuit for dividingthe signalf by the signal f to obtain a signalf, related to the meanfluid level.

4. An apparatus according to claim 3, in which the pressure gaugelocated at a low level in the container and the pressure gauge locatedat a point above the surface of the liquid each consist of a singledifferential pressure gauge.

5. An apparatus according to claim 3 wherein there are two pressuregauges located at a low level in the container and two pressure gaugeslocated at a point above the surface of the liquid, said gauges beingpaired to form two differential pressure gauges, each with one inputconnected to a point located at a low level in the container in theproximity of, respectively, an upright bounding wall and an uprightbounding wall opposite the former wall, and with the other inputconnected to a point in the container located above the surface oftheliquid.

1. A process for continuously determining the mean level of a liquidpresent in a container which can move, comprising: a. measuring the meanpressure exerted by the liquid in the vertical direction near the bottomof the container and converting it to a corresponding signal fa whereinthe mean pressure is determined by measuring the difference between thepressure in a single point located at a low level in the center of thecontainer and the pressure in a point in the container located above thesurface of the liquid; b. measuring the force exerted by a constant massin the vertical direction, which mass makes the same movements as thecontainer, converting the measured corresponding signal fc, dividing thesignal fa by signal fc to obtain a signal related to the mean level ofthe liquid in the container.
 2. A process according to claim 1, in whichprocess the mean pressure is derived from pressures measured in pointslocated at a low level in the container, in the proximity of twoopposite, upright bounding walls of the container, and from the pressurein a point in the container located above the surface of the liquidwherein the two opposite upright bounding walls are those at which theliquid in the container rises highest as a result of the movements ofthe container.
 3. An apparatus for determining the mean level of aliquid in a container subject to motion comprising: a. at least onepressure gauge connected to a point located at a low level in thecontainer; b. at least one pressure gauge connected to a point in thecontainer located above the surface of the liquid, c. a constant massconnected to a pressure gauge for measuring the pressure exerted by themass in the vertical direction, which results in a corresponding signalfc, d. a computing element for determining the difference between themeasurements made by the first and second pressure gauGes to producesignal fa, e. a computing circuit for dividing the signal fa by thesignal fc to obtain a signal fn related to the mean fluid level.
 4. Anapparatus according to claim 3, in which the pressure gauge located at alow level in the container and the pressure gauge located at a pointabove the surface of the liquid each consist of a single differentialpressure gauge.
 5. An apparatus according to claim 3 wherein there aretwo pressure gauges located at a low level in the container and twopressure gauges located at a point above the surface of the liquid, saidgauges being paired to form two differential pressure gauges, each withone input connected to a point located at a low level in the containerin the proximity of, respectively, an upright bounding wall and anupright bounding wall opposite the former wall, and with the other inputconnected to a point in the container located above the surface of theliquid.